Although the testes are absent, the male external genitalia, such as the penis and scrotum, is otherwise normal. This suggests that there was normal testicular function in early fetal life (when the baby was developing in the womb) and normal male differentiation took place (the process by which the developing baby's sex is determined). The testes are presumed, therefore, to have regressed (development stopped) for some reason. Torsion of the testes (twisting) in fetal life has been suggested as a cause.
Boys may also present with underdeveloped non-functioning testes, which can be felt in the scrotum. This is quite a different condition from congenital absence of the testes and it is called bilateral undescended testes.